This invention relates generally to the fabrication of bodies of semiconductor materials. The invention particularly relates to production of formed articles of semiconductor material which are useful in diffusion doping processes, epitaxial growth of semiconductor material, and chemical vapor deposition processes for pure polysilicon production and other related deposition methods widely used in the semiconductor industry.
One of the main applications of tubular bodies of semiconductor material, particularly of silicon, is the processing vessel for the manufacturing of semiconductor components, especially in the manufacture of epitaxial layers through a transport reaction, and in the doping process of semiconductor wafer in a diffusion furnace. In such a processing furnace, semiconductor crystals are disposed in the interior of the tube and heated up together with the tube to a desired temperature at which the doping or epitaxial precipitation process takes place.
There is another emerging application of silicon tubes in the fabrication of pure polysilicon by chemical vapor deposition (CVD) method. According to a new process disclosed by Chandra et al. (U.S. patent application Ser. No. 09/642,735), silicon tubes with large surface area are used as the deposition substrates to replace the silicon slim rod used conventionally in a CVD reactor for polysilicon production. The major benefit of using silicon tubes in the reactor is the significant increase in the production rate, especially during the initial deposition period. This new technology calls for an economical way of producing the starting silicon tubes.
The above mentioned semiconductor tubes, both for diffusion furnaces and CVD reactors, would seem to be relatively large, in the planned fabrication of semiconductors. The size of the semiconductor tube or ampoule is designated according to the diameter and the number of the wafers to be processed. For 125 mm diameter wafers, for example, the tube can have an inner diameter of about 160 mm, with a wall thickness of about 8 mm and the tube length of about 2000 mm. Both applications require a stringent high purity of the semiconductor tubes, as generally expected for semiconductor grade materials. The wall of the tube should be gas tight to prevent any leakage of the reactive gases. This restrains any form of cracks inside the wall of the semiconductor body. The tube is also supposed to be strong enough so that mechanical handling would not break or destroy it during its utilization.
The prior art of producing semiconductor bodies, particularly silicon tubes, can be roughly divided into two categories, according essentially to the deposition or growth of the semiconductor material. One is the chemical vapor deposition (CVD) of semiconductor materials, and the other one is the crystal growth through Edge-defined Film-fed Growth (EFG) method.
The CVD process is the most commonly used method for producing semiconductor bodies. In this method, a thermally decomposable gaseous semiconductor compound is brought into contact with heated surfaces of a carrier member or mold, and decomposed to yield a semiconductor material which is deposited on the carrier member surfaces. After the deposition process is completed, the system is cooled and the carrier member is removed without destroying the formed semiconductor body. Variations of this method differ only in the technique of removing the carrier member, which is mostly made of graphite according to the related literature, although the use of metallic carrier members were also reported, for example, tantalum in U.S. Pat. No. 3,139,363.
Methods for removing graphite mold include burning out the graphite material, dissolving graphite in fuming nitric or chromosulfuric acid, see for example U.S. Pat. No. 3,900,039, and pulling the carrier member out of the resulting cooled semiconductor body by forming fissures or cracks at the initial deposition stage at a elevated temperature, see also U.S. Pat. No. 3,686,378, or by depositing three successive layers of SiO2, amorphous silicon, and polycrystalline silicon, as described in U.S. Pat. No. 3,867,497.
The major problem related to the above-mentioned CVD method is the extremely high cost of the process, both on deposition and mandrel removal. The high deposition temperature, about 800-1200xc2x0 C., limits the selection of the mold materials which need to be refractory. Although techniques for a reusable carrier member, which could reduce the overall cost of the tubes, have been reported, problems related to the tubes made therefrom, such as the complexity of the associated process, and leaks during the utilization due to the minute discontinuities in the semiconductor body, are still formidable.
The EFG method for producing silicon tubes is a technique or method invented and developed by La Belle, see U.S. Pat. No. 3,591,348, and has been applied mainly for solar cell manufacturing. This technique employs a shaped crucible, which acts as a shaping die with capillary slots built into the walls, and produces monocrystalline silicon ribbons and tubes with different shapes. It was explored recently by Chandra et al, in U.S. patent application Ser. No. 09/642,735, as an approach to produce the starting silicon tubes used in their new CVD reactors.
The EFG crystal growth technique is a high temperature process with melting and freezing of silicon material. High thermal stress can build up inside the tube, which leads to easy breakage of the tube. The tube wall is generally thin and very brittle. These drawbacks of the EFG tubes preclude their application in the diffusion furnaces.
In the disclosure that follows, we present a new method for the forming of semiconductor articles, particularly silicon tubes for the above mentioned applications. This new method applies the general thermal spray technique, which has seen an extensive application in coating and net-shape forming of metal and ceramic materials. Readers may find instructive an article written by Herman, entitled xe2x80x9cPlasma-sprayed Coatingsxe2x80x9d, Scientific American, vol. 256, no. 9, September, 1998, pp. 112-117. However, no application of this technique for the forming of pure semiconductor articles, such as high purity silicon or germanium tubes, has to our knowledge been reported.
The present invention is directed to a method for producing formed semiconductor bodies, particularly full form bodies of silicon and germanium, whether pure or doped, which are used either as process vessels in diffusion furnaces for semiconductor devices or as starting substrates in chemical vapor deposition reactors for polysilicon or germanium manufacturing. In this method, a thermal spray torch, an arc plasma torch, for example, is used to generate high temperature and high-speed gas jet. Semiconductor materials that are usually in powder form are fed into the jet. Powder particles are melted/softened and accelerated by the jet, and thereafter, impact and deposit on a pre-shaped mandrel to form the desired coating layer of the semiconductor. The coating layer formed this way is, thereafter, separated from the supporting mandrel either by pulling out the mandrel mechanically or by dissolving the mandrel material into liquid chemicals or depleting the mandrel with gaseous oxidants.
Two of the major advantages of this method are the high production rate and low cost per tube comparing to both the CVD and crystal growth methods. Moreover, unlike the mandrel materials used in CVD deposition, which are limited by the high deposition temperatures, typically about 800-1200xc2x0 C., there is much broader selection of the mandrel materials for the thermal spray deposition process because the deposition temperature can be much lower, less than 400xc2x0 C., or even 200xc2x0 C., and can be easily manipulated.
Another benefit of the lower deposition temperatures is the weaker adhesion between the coating layer and the mandrel surface, which leads to easier separation of the formed article from the mandrel. Also, the thermal stress inside the spray formed body is much smaller than that formed at high deposition temperature, such as in a CVD reactor or crystal growth from melt. The thermal spray system lends itself to deposit more than one material simultaneously, which opens a venue for putting dopant into the semiconductor body.
It is, therefore, an object of the invention to provide a method for the fabrication of shaped bodies of semiconductor materials, including tubular body shapes.
Another object of the invention is to provide a method for fabrication of semiconductor bodies with high purity.
A further object of the present invention is to provide a method to form these bodies on a supporting mandrel, and to be further able to release the body from the mandrel without endangering the soundness or quality of the semiconductor body.
A yet further object of the invention is to confine mandrel temperature exposure to not more than 400xc2x0 C. and preferably not more than 200xc2x0 C. during the formation process.
A still yet further object is to provide for the addition of dopants in the materials of which the semiconductor body is formed.
Other objects, advantages and preferred embodiments of the invention will be apparent from the following detailed description.